High-speed three-dimensional (3D) imaging is an increasingly important function in advanced sensors in both military and civilian applications. For example, high-speed 3D capabilities offer many military systems with greatly increased capabilities in target detection, identification, classification, tracking, and kill determination. As a further example, real time 3D imaging techniques also have great potential in commercial applications, ranging from 3D television, virtual reality, 3D modeling and simulation, Internet applications, industrial inspection, vehicle navigation, robotics and tele-operation, to medical imaging, dental measurement, as well as apparel and footwear industries, just to name a few.
A number of three dimensional surface profile imaging methods and apparatuses described in U.S. Pat. Nos. 5,675,407; U.S. Pat. No. 6,028,672; and U.S. Pat. No. 6,147,760; the disclosures of which is incorporated herein by reference in their entireties, conduct imaging by projecting light through a linear variable wavelength filter (LVWF), thereby projecting light having a known, spatially distributed wavelength spectrum on the objects being imaged. The LVWF is a rectangular optical glass plate coated with a color-filtering film that gradually varies in color, (i.e., wavelength). If the color spectrum of a LVWF is within the visible light region, one edge of the filter rectangle may correspond to the shortest visible wavelength (i.e. blue or violet) while the opposite edge may correspond to the longest visible wavelength, (i.e. red). The wavelength of light passing through the coated color-filtering layer is linearly proportional to the distance between the position on the filter glass where the light passes and the blue or red edge. Consequently, the color of the light is directly related to the angle θ, shown in FIG. 1, at which the light leaves the rainbow projector and LVWF.
Referring to FIGS. 1 and 2 in more detail, the imaging method and apparatus is based on the triangulation principle and the relationship between a light projector (100) that projects through the LVWF (101), a camera (102), and the object or scene being imaged (104). As shown in FIG. 1, a triangle is uniquely defined by the angles theta (θ) and alpha (α), and the length of the baseline (B). With known values for θ, α, and β, the distance (i.e., the range R) between the camera (102) and a point Q on the object's surface can be easily calculated. Because the baseline B is predetermined by the relative positions of the light projector (100) and the camera (102), and the value of a can be calculated from the camera's geometry, the key to the triangulation method is to determine the projection angle, θ, from an image captured by the camera (102) and more particularly to determine all θ angles corresponding to all the visible points on an object's surface in order to obtain a full-frame 3D image in one snapshot.
FIG. 2 is a more detailed version of FIG. 1 and illustrates the manner in which all visible points on the object's surface (104) are obtained via the triangulation method. As can be seen in the figure, the light projector (100) generates a fan beam of light (200). The fan beam (200) is broad spectrum light (i.e., white light) which passes through the LVWF (101) to illuminate one or more three-dimensional objects (104) in the scene with a pattern of light rays possessing a rainbow-like spectrum distribution. The fan beam of light (200) is composed of multiple vertical planes of light (202), or “light sheets”, each plane having a given projection angle and wavelength. Because of the fixed geometric relationship among the light source (100), the lens of the camera (102), and the LVWF (101), there exists a one-to-one correspondence between the projection angle (θ) of the vertical plane of light and the wavelength (λ) of the light ray. Note that although the wavelength variations are shown in FIG. 2 to occur from side to side across the object (104) being imaged, it will be understood by those skilled in the art that the variations in wavelength could also be made from top to bottom across the object (104) or scene being imaged.
The light reflected from the object (104) surface is then detected by the camera (102). If a visible spectrum range LVWF (400-700 nm) is used, the color detected by the camera pixels is determined by the proportion of its primary color Red, Green, and Blue components (RGB). The color spectrum of each pixel has a one-to-one correspondence with the projection angle (θ) of the plane of light due to the fixed geometry of the camera (102) lens and the LVWF (101) characteristics. Therefore, the color of light received by the camera (102) can be used to determine the angle θ at which that light left the light projector (100) through the LVWF (101).
As described above, the angle α is determined by the physical relationship between the camera (102) and the coordinates of each pixel on the camera's imaging plane. The baseline B between the camera's (102) focal point and the center of the cylindrical lens of the light projector (100) is fixed and known. Given the value for angles α and θ, together with the known baseline length B, all necessary information is provided to easily determine the full frame of three-dimensional range values (x,y,z) for any and every visible spot on the surface of the objects (104) seen by the camera (102).
While the camera (102) illustrated in FIG. 2 effectively produces full frame three-dimensional range values for any and every visible spot on the surface of an object (104), the camera (102) also requires a high signal-to-noise (S/N) ratio, a color sensor, and an LVWF (101) with precision spectral variation, all of which is expensive to achieve. Consequently, there is a need in the art for an inexpensive yet high speed three dimensional camera.